Digital optical transmitter

ABSTRACT

A cable television system ( 100 ) having forward and reverse paths includes, in the reverse path, a digital optical transmitter ( 200 ) for receiving an RF signal, converting it to a digital signal, and adding a digital pilot tone thereto. A laser is driven in accordance with the summed digital signal to generate a digital optical signal representative of the pilot tone and the RF signal. The cable television system ( 100 ) also includes an optical receiver ( 305 ) for receiving the digital optical signal and recovering therefrom the RF signal and the pilot tone. The optical transmitter ( 200 ) and receiver ( 305 ) are coupled by fiber optic communication media ( 110 ).

FIELD OF THE INVENTION

[0001] This invention relates generally to fiber optic communications,and more specifically to optical transmitters for use in fiber opticcommunications.

BACKGROUND OF THE INVENTION

[0002] Cable television systems typically include a headend section forreceiving satellite signals and demodulating the signals to baseband.The baseband signal is then converted to an optical signal fortransmission from the headend section over fiber optic cable. Opticaltransmitters are distributed throughout the cable system for splittingand transmitting optical signals, and optical receivers are provided forreceiving the optical signals and converting them to radio frequency(RF) signals that are further transmitted along branches of the systemover coaxial cable rather than fiber optic cable. Taps are situatedalong the coaxial cable to tap off the cable signals to subscribers ofthe system.

[0003] Various factors influence the ability to accurately transmit andreceive optical signals within a cable television system. As the lengthof fiber optic cable within a system increases, for example, signallosses also increase. Furthermore, temperature fluctuations, which causevariation in the optical modulation index of the optical transmitter,can result in variation of the radio frequency (RF) output level of theoptical receiver. Signal distortions can be caused by non-linearities inthe laser and photodiode of the optical transmitter.

[0004] Although these problems can be mitigated by employing expensivetechniques, e.g., decreasing fiber lengths between optical nodes, suchtechniques may prohibitively increase costs to both subscribers andservice providers. Thus, what is needed is a better way to providereliable and accurate transmission of optical signals within a cabletelevision system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIG. 1 is a block diagram of a cable television system inaccordance with the present invention.

[0006]FIG. 2 is an electrical block diagram of an optical transmitterincluded in the cable television system of FIG. 1 in accordance with thepresent invention.

[0007]FIG. 3 is an electrical block diagram of an optical receiverincluded in the cable television system of FIG. 1 in accordance with thepresent invention.

[0008]FIG. 4 is a block diagram of a cable television having multipleoutputs to subscriber regions in accordance with the present invention.

[0009]FIG. 5 is an electrical block diagram of an optical transmitterincluded in the cable television system of FIG. 4 in accordance with thepresent invention.

[0010]FIG. 6 is an electrical block diagram of an optical receiverincluded in the cable television system of FIG. 4 in accordance with thepresent invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0011]FIG. 1 shows a communications system, such as a cable televisionsystem 100 having both forward and reverse paths, i.e., having theability to communicate downstream in the forward direction and upstreamin the reverse direction. The cable television system 100 includes aheadend 105 for receiving satellite signals that are demodulated tobaseband or an intermediate frequency (IF). The baseband signal is thenconverted to cable television signals that are routed throughout thesystem 100 to subscriber equipment 130, such as set top decoders,televisions, or computers, located in the residences or offices ofsystem subscribers. The headend 105 can, for instance, convert thebaseband signal to an optical signal that is transmitted over fiberoptic cable 110, in which case a remotely located optical node 115converts the optical signal to an electrical radio frequency (RF) signalfor further transmission through the system 100 over coaxial cable 120.Taps 125 located along the cable 120 at various points in thedistribution system split off portions of the RF signal for routing tosubscriber equipment 130 coupled to subscriber drops provided at thetaps 125.

[0012] The system 100, as mentioned, also has reverse transmissioncapability so that signals, such as data, video, or voice signals,generated by the subscriber equipment 130 can be provided back to theheadend 105 for processing. The reverse signals travel through the taps125 and any nodes 115 and other cable television equipment, e.g.,reverse amplifiers, to the headend 105. In the configuration shown inFIG. 1, RF signals generated by the subscriber equipment 130 travel tothe node 115, which converts the RF signals to optical signals fortransmission over the fiber optic cable 110 to the headend 105.

[0013] Referring to FIG. 2, a digital reverse transmitter 200 isprovided for transmitting digital optical signals to the headend 105 inthe reverse direction. The transmitter 200 can, for instance, beincluded within the optical node 115, although other locations withinthe cable television system 100 may also include the digital reversetransmitter 200 of the present invention. The transmitter 200 receives,at an input 202, an analog information signal that is representative ofone or more reverse RF signals from the subscriber equipment 130. At itsoutput 204, the transmitter 200 provides a digital optical signal thatis generated in accordance with the analog information signal as well asan optional pilot tone that serves to provide a reference level duringprocessing at the headend 105.

[0014] More specifically, the digital reverse transmitter 200 includesan analog-to-digital (A/D) converter 205 for converting the analog inputto a digital signal, i.e., a digital word comprising a particular numberof bits, in a conventional manner. The resolution of the A/D converter205, of course, is dependent upon transmitter design parameters. Thetransmitter 200 can also include a digital pilot tone generator 210 forproviding a digital pilot tone in the form of a number of bitsrepresentative of a particular level and frequency. The digital pilottone generator 210 could, for instance, include input switches by whichthe level and frequency could be varied. U.S. Pat. No. 5,563,815 toJones, the teachings of which are hereby incorporated by reference,shows a digital tone oscillator that could be used to implement thegenerator 210 included in the transmitter 200 of the present invention.

[0015] A summer 215 receives the digital information signal from the A/Dconverter 205 and the digital pilot tone signal from the generator 210and digitally adds the two signals by performing binary addition in aknown manner. The summed signal is then coupled to a parallel-to-serial(P/S) converter, or a serializer 220, which receives the parallel inputsrepresentative of the summed signal and converts the inputs into aserial bit stream. A laser diode 225 is then driven to generate anoptical signal in accordance with the serial bit stream. It will beappreciated that the serializer 220 can also include a driver fordriving the laser diode 225 and frame encoding circuitry for encodingthe serialized digital signal into frames of data.

[0016]FIG. 3 is a block diagram of an optical receiver 305 for receivingthe digital optical signal transmitted by the optical transmitter 200.The receiver 305 can be, for instance, located in the headend 105,although other locations, such as any intervening nodes, may also employthe receiver 305. The receiver 305 includes a detector, such as aphotodiode 310, for receiving the digital optical signal transmittedover the fiber optic cable 110 and generating therefrom a serial streamof electrical pulses in accordance with the optical signal. The outputsignals provided by the photodiode 310 are coupled to aserial-to-parallel (P/S) converter 315 for generating therefrom a set ofparallel outputs corresponding to a digital word. The receiver 305further includes a digital-to-analog (D/A) converter 320 for convertingthe signal provided at its digital input to an analog signal in a knownmanner. Thereafter, the analog signal is processed by a filter 325 toseparate the pilot tone signal from the information signal. Morespecifically, the filter 325 preferably comprises a low pass filter thatonly passes the fundamental frequency component of the output of the D/Aconverter 320. As a result, the digital optical receiver 305 is able toprovide at its output a reference signal, i.e., the pilot tone, and ananalog signal that approximates the analog information signal initiallyprovided to the optical transmitter 200. Furthermore, this can be donewithout encountering many of the problems that arise in prior artdesigns.

[0017] In conventional cable television systems, optical links in thereverse path use amplitude modulation to directly modulate a lasergenerating a reverse optical signal. As a result, RF output level of theoptical receiver is directly dependent upon the optical modulation index(OMI), which in turn is directly related to the RF drive current, thelaser threshold current, and the laser bias current of the laser locatedin the transmitter. Since the laser bias and threshold currents varywith temperature, which in turn causes temperature variations of theOMI, the RF output level of the optical receiver also varies withtemperature. However, the laser within the transmitter 200 of thepresent invention is digitally modulated so that the RF levelinformation is encoded according to a bit stream; as a result,variations in the OMI, the laser bias current, the laser thresholdcurrent, and the temperature do not affect RF output levels of theoptical receiver 305.

[0018] Prior art optical transmission that use AM modulation also resultin a system in which the linearity of the received optical signal isdirectly dependent upon the linearity of the transmitting laser and thereceiving photodiode. Therefore, non-linearities of those devices cangreatly degrade the performance of the reverse path system.Additionally, the non-linear conversion processes of lasers andphotodiodes in conventional systems vary with temperature, thus furtherdegrading the performance. Conversely, the digital optical system, i.e.,the digital optical transmitter 200 and the digital optical receiver305, of the present invention only generates and resolves two amplitudelevels rather than a continuum of levels. As a result, linearityrequirements of the laser and photodiode are reduced, which results inbetter performance and less expense.

[0019] Another problem associated with conventional cable televisionsystems is that reverse pilot tones are seldom used due to thecomplications and costs. When such pilot tones are used, an additionaloscillator, which is not digital, is generally located outside thetransmitter and is susceptible to temperature variations. The oscillatorsignal is combined with the analog RF signal, and the combined signal isused to modulate the laser diode current to provide an optical output.Prior art pilot tones are used by an optical transmitter to ensure thatthere is always a minimum RF signal modulating the laser, therebydecreasing the spurious noise generated by the laser, and by an opticalreceiver for gain control purposes. However, since oscillator outputlevel drifts with temperature, the RF output level of the opticalreceiver will also drift with temperature so that gain control isessentially useless. As mentioned above, use of the combined digitalpilot tone and digital information signal according to the presentinvention solves the prior art temperature dependency problems. At thesame time, the digital pilot tone can be used by the transmitter 200 tomodulate the laser even when no RF input is present.

[0020] Still another advantage of the digital optical transmitter 200and receiver 305 of the present invention is that the cable system 100can, without significant cost or performance penalties, employ anarchitecture in which fiber stretches deeper into the system 100. As acable television signal travels along a fiber optic cable 110, thesignal decreases in power as a result of laser noise, Rayleighbackscattering, photodiode shot noise, receiver amplifier noise,unmodulated Fabry-Perot sporadic noise, and post amplifier intrinsicnoise. These factors cause the carrier-to-noise ratio (CNR) to decrease.Conventionally, this problem is mitigated by driving the transmitterlaser with more power and/or increasing the receive sensitivity of thereceiver photodiode at great expense. However, this need not be done ina system 100 according to the present invention since the noise sourcesand corresponding signal degradation resulting from increased fiberlengths does not affect recovery of information to the same extent as inprior art systems.

[0021] Referring next to FIG. 4, a modified cable television system 400is depicted. The system 400 includes a headend 105 for generating cabletelevision signals that are split off to subscriber equipment 130 bytaps 125. However, in the system 400, the optical node 415 splits offthe downstream cable signal for transmission to multiple distributionsystems 430, 435, or branches. Each branch typically provides service tosubscribers located in different geographic regions. Upstream reversesignals provided by subscriber equipment 130 in the different branches430, 435 is transmitted in the form of analog RF signals to the opticalnode 415, which combines the signals for further upstream transmissionin the form of an optical signal. According to the present invention,the upstream signals from the different branches 430, 435 can beconverted to a digital optical signal in a manner that minimizes oreliminates many of the problems associated with prior art cabletelevision systems.

[0022]FIG. 5 is an electrical block diagram of an optical transmitter500 that can, in accordance with the present invention, be used toprocess multiple analog inputs. At input 502, the transmitter 500receives a first analog input, such as from a first branch 430 of acable television system 400, and, at input 503, the transmitter 500receives a second analog input, such as from a second branch 435 of thesystem 400. First and second A/D converters 205, 505 respectivelyconvert the received RF signals to digital information signals that areseparately summed, by summers 215, 515, with the digital pilot tone.Each summed signal is then serialized by serializers 220, 520 to resultin first and second serial bit streams that are representative of thefirst and second RF signals, respectively, as separately combined withthe digital pilot tone. According to the present invention, bits of theserial bit streams are interleaved by an interleaver 550 to form asingle digital signal that modulates the laser diode 225. As a result, asingle digital optical signal can be provided at the output 504 of thetransmitter 500.

[0023] Referring to FIG. 6, an optical receiver 605 for processing thedigital optical signal generated by the transmitter 500 is shown. Thereceiver 605 includes a photodiode 310 for generating electrical pulsesfrom the optical signal and a deinterleaver 650 for deinterleaving thesignal comprising the electrical pulses. Once the deinterleaver 650 hasseparated the received signal into separate serial bit streams, theoutputs are coupled to first and second S/P converters 315, 615, firstand second D/A converters 320, 620, and first and second filters 325,625 to recover approximations of the pilot tone and the RF signals thatwere provided to the transmitter 500.

[0024] It will be appreciated that the interleaver 550 and thedeinterleaver 650 can be implemented using conventional components.Typically, the interleaver 550 could be a framing device capable ofimplementing a time-domain-multiplexing (TDM) scheme with respect to theincoming bit streams. In such an implementation, a frame clock (notshown) would be coupled to the interleaver 550, and one frame wouldconsist of a number of sub-frames equivalent to the number of incomingbit streams. A flag bit would likely be inserted into the frame foridentifying the start of the frame. The deinterleaver 650 is capable ofextracting the frame clock signal from the incoming information andrecognizing the flag bits indicative of frame starts. Each bit wouldthen be routed to its respective bit stream to recover the originalsignals.

[0025] Although only two input branches into the transmitter 500 and twoprocessing paths through the transmitter 500 and the receiver 605 areshown, a plurality of paths can be provided depending upon the number ofincoming analog signals to be processed by the transmitter 500. Forexample, if five RF signals are traveling in the reverse paths of fivebranches of a cable television system, the optical transmission systemaccording to the present invention would individually convert eachreverse signal to a digital signal, add it to the pilot tone, andserialize the combined signal. All serialized signals would then becombined by the interleaver 550 to generate a bit stream for modulatingthe laser diode 225 (FIG. 5). On the receiver end, the deinterleaver 650would deinterleave the received digital optical signal to provide fiveserial signals that would be individually processed by S/P converters,D/A converters, and filters to provide five analog outputs as well as anapproximation of the pilot tone.

[0026] In this manner, reverse signals of the same frequency can beconveniently sent to the headend 105 over the same return fiber 110.This is very important since cable television systems typically onlyallocate a small amount of bandwidth, e.g., 5-40 MHZ, for return pathtransmissions, which means that varying the frequency of each returnpath signal would not be practical.

[0027] In summary, the reverse digital transmission system describedabove provides one or more reverse signals without many of the problemspresent in prior art systems. As a result, information can be sent fromsubscribers to the headend in a more reliable and less expensive manner.

What is claimed is:
 1. An optical transmitter, comprising: ananalog-to-digital (A/D) converter for converting an analog informationsignal to a digital information signal; a pilot tone generator forgenerating a digital pilot tone; a summer coupled to the A/D converterand the pilot tone generator for adding the digital pilot tone to thedigital information signal to result in a summed signal; and a laserdiode coupled to the summer for providing an optical signal inaccordance with the summed signal.
 2. The optical transmitter of claim1, further comprising: an input for coupling the analog informationsignal to the A/D converter.
 3. The optical transmitter of claim 1,further comprising: a serializer coupled to the summer and the laserdiode for converting the summed signal to a serial signal.
 4. Theoptical transmitter of claim 3, further comprising: a driver for drivingthe laser diode to generate the optical signal.
 5. The opticaltransmitter of claim 3, further comprising: an interleaver coupled tothe serializer and the laser diode for interleaving bits of the serialsignal.
 6. The optical transmitter of claim 5, further comprising:circuitry, coupled to the interleaver, for receiving another analoginformation signal, converting the other analog information signal toanother digital signal, which is summed with the digital pilot tone andserialized to result in another serial signal, and for providing theother serial signal to the interleaver, wherein the interleaverinterleaves bits of the serial signal with bits of the other serialsignal to drive the laser diode.
 7. The optical transmitter of claim 1,wherein the optical transmitter is included in a reverse path of a cabletelevision system for transmitting information from system subscribersto a headend for further processing.
 8. A cable television system forproviding signals, the cable television system comprising: an opticaltransmitter for receiving an analog information signal and transmittingin accordance therewith a digital optical signal representative of theanalog information signal; an optical receiver for receiving the digitaloptical signal and recovering therefrom an analog signal representativeof the analog information signal; and an optical communication mediumfor coupling the optical transmitter and the optical receiver.
 9. Thecable television system of claim 8, wherein the digital optical signalcomprises: a digital representation of the analog information signal;and a digital pilot tone added to the digital representation of theanalog information signal prior to transmission of the digital opticalsignal, wherein the optical receiver further recovers a pilot tone levelfrom the digital optical signal.
 10. The cable television system ofclaim 9, wherein the optical transmitter comprises: an analog-to-digital(A/D) converter for converting the analog information signal to adigital information signal; a pilot tone generator for generating thedigital pilot tone; a summer coupled to the A/D converter and the pilottone generator for adding the digital pilot tone to the digitalinformation signal to result in a summed signal; and a laser diodecoupled to the summer for providing the digital optical signal inaccordance with the summed signal.
 11. The cable television system ofclaim 10, wherein the optical transmitter further comprises: aserializer coupled to the summer and the laser diode for converting thesummed signal to a serial signal.
 12. The cable television system ofclaim 11, wherein the optical transmitter further comprises: aninterleaver coupled to the serializer and the laser diode forinterleaving bits of the serial signal.
 13. The cable television systemof claim 12, wherein the optical transmitter further comprises:circuitry, coupled to the interleaver, for receiving another analoginformation signal, converting the other analog information signal toanother digital signal, which is summed with the digital pilot tone andserialized to result in another serial signal, and for providing theother serial signal to the interleaver, wherein the interleaverinterleaves bits of the serial signal with bits of the other serialsignal to drive the laser diode.
 14. The cable television system ofclaim 9, wherein the optical receiver comprises: a detector forconverting the digital optical signal to a digital electrical signal; adigital-to-analog (D/A) converter for converting the digital electricalsignal to an analog electrical signal; and a filter for processing theanalog electrical signal to recover therefrom the pilot tone and theanalog signal that is representative of the analog information signal.15. The cable television system of claim 9, further comprising: aforward path for transmitting information to system subscribers; and areverse path for receiving information from system subscribers, whereinthe reverse path includes the optical transmitter and the opticalreceiver.
 16. A cable television system having forward and reverse pathsfor respectively transmitting forward and reverse signals, the cabletelevision system comprising: first and second distribution systems forproviding information to system subscribers located, respectively, infirst and second geographic regions; a headend for generating andtransmitting optical signals; an optical node, remotely located from theheadend, for converting the optical signals to electrical signals andfor transmitting the electrical signals over the first and seconddistribution systems, wherein, in the reverse path, the optical nodecomprises an optical transmitter, including: a first A/D converter forreceiving a first analog information signal from subscriber equipmentincluded in the first distribution system and converting the firstanalog information signal to a first digital information signal; asecond A/D converter for receiving a second analog information signalfrom subscriber equipment included in the second distribution system andconverting the second analog information signal to a second digitalinformation signal; a digital pilot tone generator for generating adigital pilot tone; summers coupled to the first and second A/Dconverters and the digital pilot tone generator for adding the digitalpilot tone to the first digital information signal to result in a firstsummed signal and for adding the digital pilot tone to the seconddigital information signal to result in a second summed signal;serializers coupled to the summers to convert the first and secondsummed signals to first and second serial signals; an interleavercoupled to the serializers for interleaving bits of the first summedsignal with bits of the second summed signal to generate an interleavedsignal; and a laser diode coupled to the interleaver for emitting adigital optical signal in accordance with the interleaved signal.